1. Field of the Invention
The present invention relates to manganese dioxide for lithium primary batteries and a process for producing the same. More particularly, it relates to an improvement of a cathode active material of a lithium primary battery comprising lithium or a lithium alloy as an anode active material.
2. Description of the Related Art
Typical cathode active materials known for lithium primary batteries comprising lithium or a lithium alloy as an anode active material include manganese dioxide, carbon fluorid, etc., which have been put to practical use.
Manganese dioxide out of these cathode active materials has been used for preference for its inexpensiveness and abundance.
Where manganese dioxide is used as a cathode active material, it is a generally followed practice that electrolytic manganese dioxide is heat treated at 350 to 430xc2x0 C. as disclosed, e.g., in Japanese Patent Publication No. 4064/82.
Electrolytic manganese dioxide to be heat treated is one having been neutralized with ammonia. Electrolytic manganese dioxide itself is useful as a cathode material in manganese dry batteries and alkaline manganese dry batteries. Ammonia-neutralized one is for manganese dry batteries, and soda-neutralized one is for alkaline manganese dry batteries. Electrolytic manganese dioxide having been neutralized with soda and heat treated contains 0.3 to 0.5% by weight of sodium. If the sodium-containing manganese dioxide is applied to a lithium primary battery, the lithium primary battery has significantly reduced discharge performance because the residual sodium in manganese dioxide contaminates the anode lithium to interfere with the electrode reaction.
For the above reason, ammonia neutralization has been adopted because ammonia evaporates together with the water content during heat treatment. It has been pointed out, however, that ammonia neutralization is accompanied by the problem of irritating smell given off on ammonia evaporation which deteriorates the working environment. In addition, an existing plant having no step of ammonia neutralization could not be applied as such to preparation of manganese dioxide for lithium primary batteries.
Ammonia-neutralized manganese dioxide has another disadvantage that it undergoes marked reduction in BET specific surface area, which leads to deterioration of low-temperature characteristics or storage characteristics when applied to lithium primary batteries.
The present invention has been made in the light of the above-described circumstances. An object of the invention is to provide manganese dioxide for lithium primary batteries and a process for producing the same which achieve improvement on working environment, reduction of sodium dissolution, and improvement on low-temperature characteristics and storage characteristics of batteries.
As a result of extensive investigation, the present inventors have found that the above object is accomplished by carrying out soda neutralization of electrolytic manganese dioxide while controlling the sodium content within a given range.
The present invention has been completed based on this finding. The present invention provides manganese dioxide with a sodium content of 0.05 to 0.2% by weight for lithium primary batteries which is obtained by soda neutralization and heat treatment of electrolytic manganese dioxide.
The present invention also provides a process for producing manganese dioxide with a sodium content of 0.05 to 0.2% by weight for lithium primary batteries comprising the steps of neutralizing electrolytic manganese dioxide with an aqueous solution containing 2.0 to 5.0 g of sodium hydroxide per kg of manganese dioxide and then heating the neutralized manganese dioxide.
The manganese dioxide for lithium primary batteries according to the present invention brings about improved on the working environment, has reduced sodium dissolution, and improves low-temperature characteristics and storage characteristics of batteries. The process according to the present invention provides such manganese dioxide on an industrial scale with satisfactory workability and ease.